1. Field of the Invention
The present invention relates generally to a data transceiver apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for transmitting and receiving data using a variable modulation technique during retransmission.
2. Description of the Related Art
Presently, the mobile communication system has evolved from an early voice-based communication system to a high-speed, high-quality radio data packet communication system for providing a data service and a multimedia service. In addition, a 3rd generation mobile communication system, divided into an asynchronous 3GPP (3rd Generation Partnership Project) system and a synchronous 3GPP2 (3rd Generation Partnership Project 2) system, is being standardized for a high-speed, high-quality radio data packet service. For example, standardization on HSDPA (High Speed Downlink Packet Access) is performed by the 3GPP, while standardization on 1xEV-DV (1xEvolution-Data and Voice) is performed by the 3GPP2. Such standardizations are implemented to determine solutions for high-speed, high-quality radio data packet transmission services of 2 Mbps or more in the 3rd generation mobile communication system. Further, a 4th generation mobile communication system has been proposed, which will provide a high-speed, high-quality multimedia service superior to that of the 3rd generation mobile communication system.
A principal factor that impedes a high-speed, high-quality radio data service lies in the radio channel environment. The radio channel environment frequently changes due to a variation in signal power caused by white noise and fading, shadowing, Doppler effect caused by the movement of and the frequent change in speed of a UE (User Equipment), and interference caused by other users and multipath signals. Therefore, in order to provide a high-speed radio data packet service, there is a need for an improved technology capable of increasing adaptability to variations in the channel environment in addition to the general technology provided for the existing 2nd or 3rd generation mobile communication system. A high-speed power control method used in the existing system also increases adaptability to variations in the channel environment. However, both the 3GPP and the 3GPP2, implementing standardization on the high-speed data packet transmission, reference the AMCS (Adaptive Modulation/Coding Scheme) and HARQ (Hybrid Automatic Repeat Request) techniques.
The AMCS is a technique for adaptively changing a modulation technique and a coding rate of a channel encoder according to a variation in the downlink channel environment. Commonly, to detect the downlink channel environment, a UE measures a signal-to-noise ratio (SNR) and transmits the SNR information to a Node B over an uplink. The Node B predicts the downlink channel environment based on the received SNR information, and designates a proper modulation technique and coding rate according to the predicted value. The modulation techniques available for the AMCS include QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM (64-ary Quadrature Amplitude Modulation), and the coding rates available for the AMCS include ½ and ¾. An AMCS system applies the high-order modulations (16QAM and 64QAM) and the high coding rate ¾ to the UE located in the vicinity of the Node B, having a good channel environment, and applies the low-order modulations (QPSK and 8PSK) and the low coding rate ½ to the UE located in a cell boundary. In addition, compared to the existing high-speed power control method, the AMCS decreases an interference signal, thereby improving the average system performance.
The HARQ is a link control technique for correcting an error by retransmitting the errored data upon an occurrence of a packet error at an initial transmission. Generally, the HARQ is classified into Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR).
CC is a technique for transmitting a packet such that the whole packet transmitted at a retransmission is equal to the packet transmitted at the initial transmission. In this technique, a receiver combines the retransmitted packet with the initially transmitted packet that is previously stored in a buffer. By doing so, it is possible to increase reliability of coded bits input to a decoder, thus resulting in an increase in the overall system performance. Combining the two same packets is similar to repeated coding in terms of the effects, so it is possible to increase a performance gain by about 3 dB on average.
FIR is a technique for transmitting a packet comprised of only redundant bits generated from the channel encoder instead of the same packet, thus improving performance of a decoder in the receiver. That is, the FIR uses the new redundant bits as well as the initially transmitted information during decoding, resulting in a decrease in the coding rate, which in turn thereby improves performance of the decoder. It is well known in coding theory that a performance gain by a low coding rate is higher than a performance gain by repeated coding. Therefore, the FIR is superior to the CC in terms of only the performance gain.
Unlike the FIR, the PIR is a technique for transmitting a combined data packet of the information bits and the new redundant bits at retransmission. Therefore, the PIR can obtain the similar effect as the CC by combining the retransmitted information bits with the initially transmitted information bits during decoding, and also obtain the similar effect as the FIR by performing the decoding using the redundant bits. The PIR has a coding rate slightly higher than that of the FIR, showing intermediate performance between the FIR and the CC. However, the HARQ should be considered in the light of not only the performance but also the system complexity, such as a buffer size and signaling of the receiver. As a result, it is difficult to determine which technique is optimal for a given system.
The AMCS and the HARQ are separate techniques for increasing adaptability to the variations in the link environment. It is possible to remarkably improve the system performance by combining the two techniques. That is, the transmitter determines by the AMCS, a modulation technique and a coding rate proper for a downlink channel condition and then transmits packet data according to the determined modulation technique and coding rate. Then, upon failure to decode the data packet transmitted by the transmitter, the receiver sends a retransmission request. Upon receipt of the retransmission request from the receiver, the Node B retransmits the data packet by the HARQ.
FIG. 1 illustrates an existing transmitter for high-speed packet data transmission, wherein it is possible to realize various AMCS techniques and HARQ techniques by controlling a channel encoder 112.
Referring to FIG. 1, the channel encoder 112 is comprised of an encoder (not shown) and a puncturer (not shown). When input data at a determined data rate is applied to an input terminal of the channel encoder 112, the encoder performs encoding in order to decrease a transmission error rate. Further, the puncturer punctures an output of the encoder according to a coding rate and an HARQ type previously determined by a controller 122, and provides its output to a channel interleaver 114. The future mobile communication system needs a powerful channel coding technique in order to reliably transmit high-speed multimedia data. The channel encoder 112, as illustrated in FIG. 2, is comprised of a turbo encoder 200 with a mother coding rate of R=⅕, a puncturer 216 and a buffer 202. It is known in the art that channel coding by a turbo encoder performs closest to the Shannon limit in terms of a bit error rate (BER) even at a low SNR. Channel coding by a turbo encoder has also been adopted for the HSDPA and 1xEV-DV standardization by the 3GPP and the 3GPP2. The output of the turbo encoder 200 can be divided into systematic bits and parity bits. The “systematic bits” refer to actual information bits to be transmitted, while the “parity bits” refer to a signal used to help a receiver correct a possible transmission error. The puncturer 216 selectively punctures the systematic bits or the parity bits output from the encoder 200, satisfying a determined coding rate. The systematic bits and the parity bits from the turbo encoder 200 are temporarily stored in the buffer 202, to be used during retransmission at a retransmission request of the receiver.
Referring to FIG. 2, upon receiving one input data frame, the turbo encoder 200 outputs the intact input data frame as a systematic bit frame X, and further outputs two different parity bit frames Y1 and Y2. In addition, the turbo encoder 200 outputs two different parity bit frames Z1 and Z2 by performing interleaving and encoding on the input data frame. The systematic bit frame X and the parity bit frames Y1, Y2, Z1 and Z2 are provided to the puncturer 216 in a transmission unit of 1, 2, . . . , N. The puncturer 216 determines a puncturing pattern according to a control signal provided from the controller 122 of FIG. 1, and performs puncturing on the systematic bit frame X, and the four different parity bit frames Y1, Y2, Z1 and Z2 using the determined puncturing pattern, thus outputting desired systematic bits S and parity bits P. Here, the buffer 202 is provided between the turbo encoder 200 and the puncturer 216 in order to facilitate realization of the HARQ. That is, when IR (Incremental Redundancy) is used as the HARQ, different coded bits must be transmitted at each retransmission. Therefore, all coded bits generated by the turbo encoder 200 at a mother code rate are stored in the buffer 202, and the stored coded bits are output according to a corresponding puncturing pattern at each retransmission. If the buffer 202 is not provided, the same coding process must be repeated by the turbo encoder 200 at each retransmission, affecting the processing time and power efficiency. However, when CC is used as the HARQ, the same data is transmitted at each retransmission. In this case, the use of the buffer 202 causes a decrease in the efficiency, so it would be more efficient to perform a retransmission process after the channel interleaver 114 of FIG. 1.
As described above, the puncturing pattern used to puncture the coded bits by the puncturer 216 depends upon the coding rate and the HARQ type. That is, using the CC, it is possible to transmit the same packet at each transmission by puncturing the coded bits such that the puncturer 216 has a fixed combination of the systematic bits and the parity bits according to a given coding rate. Using PIR, the puncturer 216 punctures the coded bits in a combination of the systematic bits and the parity bits according to the given coding rate at initial transmission, and punctures the coded symbols in a combination of various parity bits at each retransmission, thus decreasing in the overall coding rate. For example, using the CC with the coding rate of ½, the puncturer 216 can continuously output the same bits X and Y1 for one input bit at initial transmission and retransmission, by fixedly using [1 1 0 0 0 0] in the order of the coded bits [X Y1 Y2 X′ Z1 Z2] as the puncturing pattern. Using the FIR, the puncturer 216 outputs the coded bits in the order of [X1 Y11 X2 Z21] at initial transmission and in the order of [Y21 Z21 Y12 Z12] at retransmission for two input bits, by using [1 1 0 0 0 0; 1 0 0 0 0 1] and [0 0 1 0 0 1; 0 1 0 0 1 0] as the puncturing patterns at initial transmission and retransmission, respectively. Meanwhile, though not separately illustrated, the channel encoder using R=⅓ codes adopted by the 3GPP2 can be realized by the turbo encoder 200 and the puncturer 216 of FIG. 2.
A packet data transmission operation by the AMCS system and the HARQ system realized by FIG. 1 will be described herein below. Before transmission of a new packet, the controller 122 of the transmitter determines a proper modulation technique and a coding rate based on the downlink channel condition information provided from the receiver. Thereafter, the controller 122 controls the channel encoder 112, a modulator 116 and a channel demultiplexer 118 in a physical layer based on the determined modulation technique and coding rate and a predefined HARQ type. A data rate in the physical layer is determined according to the determined modulation technique and coding rate and the number of multiple codes in use. The channel encoder 112, under the control of the controller 122, performs coding by the turbo encoder 200 and performs bit puncturing by the puncturer 216 according to a given puncturing patter, thereby outputting coded bits. The coded bits output from the channel encoder 112 are provided to the channel interleaver 114, where they are subject to interleaving. Interleaving is a technique for preventing a burst error by randomizing the input bits to disperse data symbols into several places instead of concentrating the data symbols in the same place in a fading environment. For ease of explanation, the size of the channel interleaver 114 is assumed to be greater than or equal to the total number of the coded bits. The modulator 116 symbol-maps the interleaved coded bits according to the modulation technique previously determined by the controller 122 and a given symbol mapping technique. If the modulation technique is represented by M, the number of coded bits constituting one symbol becomes log2M. Shown in Table 1 are modulation techniques used in the AMCS and the numbers of bits constituting one symbol.
TABLE 1modulation type (M)number of bits constituting one symbol (log2M)QPSK216QAM464QAM6
The channel demultiplexer 118 demultiplexes a symbol received from the modulator 116 into as many symbols as the number of multiple codes assigned by the controller 122 for high-speed data symbol transmission at a data rate determined by the controller 122. A spreader 120 spreads the demultiplexed symbols from the channel demultiplexer 118 with the assigned multiple codes. The multiple codes may include Walsh codes for identifying channels. When a fixed chip rate and a fixed spreading factor (SF) are used in the high-speed packet transmission system, the rate of symbols transmitted with one Walsh code is constant. Therefore, in order to use the determined data rate, it is necessary to use multiple Walsh codes. For example, when a system, using a chip rate of 3.84 Mcps and an SF of 16 chips/symbol, uses 16QAM and a channel coding rate of ¾, a data rate that can be provided with one Walsh code becomes 1.08 Mbps. Therefore, when 10 Walsh codes are used, it is possible to transmit data at a data rate of a maximum of 10.8 Mbps.
FIG. 3 illustrates a structure of a receiver corresponding to the transmitter of FIG. 1. Referring to FIG. 3, a despreader 312 despreads received data according to information on the multiple codes in use, the information being provided through signaling. A channel multiplexer 314 multiplexes the despread received data and provides its output to a demodulator 316. The demodulator 316 performs demodulation corresponding to the modulation used by the transmitter, and provides LLR (Log Likelihood Ratio) values for the symbols to a deinterleaver 318. The deinterleaver 318, having a structure corresponding to that of the interleaver 114 of FIG. 1, performs deinterleaving on the demodulated data and restores the original data sequence. The deinterleaved data is provided to a combiner 320, where it is combined with the same previously received data in a bit unit. If CC is used as the HARQ, the same data is transmitted at each retransmission. In this case, since combining can be performed using one buffer, a buffer controller 322 is unnecessary. However, if IR is used as the HARQ, a different redundancy packet may be transmitted at retransmission, so the buffer controller 322 is necessary. The buffer controller 322 properly assigns buffers in the combiner 320 to the received data so that the received data can be combined with the same previously received data. An output of the combiner 320 is provided to a channel decoder 324. The channel decoder 324 performs decoding on the output of the combiner 320, checks a CRC error for the received data, and transmits a NACK or ACK signal to a transmitter according to the CRC check result. Upon receiving the NACK signal from the receiver, the transmitter performs the re-transmission process according to the HARQ. However, upon receiving the ACK signal from the receiver, the transmitter begins transmission of a new data packet.
In the transmitter of the high-speed packet transmission system of FIG. 1, it is assumed that the AMCS defined by the controller 122 at initial transmission of a data packet according to a channel environment is applied even during retransmission without modification. However, as described above, a high-speed data transmission channel may subject to a change in channel environment even for an HARQ period due to the change in the number of UEs in a cell and the Doppler shift. Therefore, maintaining the modulation technique and the coding rate used at the initial transmission contributes to a reduction in the system performance. For this reason, the ongoing HSDPA and 1xEV-DV standardizations consider using the AMCS even at retransmission.
As an example, a new technique capable of changing both a modulation technique and a coding rate at retransmission has been proposed. Commonly, the size of transmittable data is changed according to a modulation technique and a coding rate, so the proposed new technique enables transmission of data by changing TTI (Time To Interleaving), a minimum unit of processing packet data. Therefore, the new technique is advantageous in that it can adjust to variations in the channel environment. However, the use of the variable TTI increases complexity of realization and signaling. Further, this technique supports only the IR among the HARQ types.
As another example, in a system wherein CC is used as the HARQ and a coding rate at retransmission is identical to a coding rate at initial transmission, if the number of available codes changes, another proposed technique changes a modulation technique for the retransmission to adapt to the change, and retransmits a part or all of the initially transmitted packet according to the changed modulation technique. Meanwhile, a receiver partially combines the retransmitted partial packet with the initially transmitted full packet, resulting in a decrease in the entire BER of a decoder. This technique, as it uses a fixed TTI and has a characteristic of partial Chase combining, is advantageous in that its realization and signaling is simple. Although this technique can decrease BER by retransmitting an unspecified part of the randomly interleaved data and combining the retransmitted partial data with the initially transmitted full packet, an improvement in a frame error rate (FER) is restrictive. In addition, this technique can support only CC among the HARQ types.
Therefore, in a communication system based on a fixed TTI, there have been demands for one method for changing a modulation technique during retransmission regardless of the HARQ type in use even though the number of available codes remains unchanged, and another method for improving system performance by properly selecting a transmission packet according to the changed modulation technique.